1. Field of the Invention
The present invention relates to a switching power supply unit and an electronic apparatus incorporating the switching power supply unit.
2. Description of the Related Art
Typically, in a switching power supply unit, a photocoupler including a light-emitting device and a phototransistor is used for providing feedback from the secondary side to the primary side while also providing insulation between the primary side and the secondary side. In that case, the light-emitting device is connected to an output voltage detecting circuit connected to a rectifying circuit on the secondary side, and the phototransistor is connected to a control circuit connected to a control terminal of a switching element on the primary side. By using the photocoupler arranged to provide feedback from the secondary side to the primary side as described above, short-circuiting between the primary side and the secondary side is prevented even if damage to the feedback circuitry occurs.
Due to the recent development of wireless devices such as cellular phones, electronic apparatuses incorporating switching power supply units have experienced problems in that electromagnetic waves emanating from cellular phones and electronic apparatuses, cause an incorrect operation of a photocoupler and in turn cause an incorrect operation of a power supply, resulting in various incorrect operations, such as a reset, of the electronic apparatuses.
In order to overcome the problems described above, preferred embodiments of the present invention provide a switching power supply unit and an electronic apparatus incorporating such a power supply unit, which are not susceptible to incorrect operations caused by electromagnetic waves.
According to one preferred embodiment of the present invention, a switching power supply unit includes a transformer having at least a primary winding and a secondary winding, a switching element connected in series with the primary winding, a control circuit connected to a control terminal of the switching element, a rectifying circuit connected to the secondary winding, an output voltage detecting circuit connected to the rectifying circuit, a photocoupler which feeds an output voltage detected by the output voltage detecting circuit back to the control circuit, and a shield member arranged to electromagnetically shield the photocoupler.
The photocoupler may be covered by the shield member on the top side thereof. Also, the photocoupler may be surrounded by the shield member.
Furthermore, the shield member may be a heat sink made of metal.
According to another preferred embodiment of the present invention, an electronic apparatus includes the switching power supply unit according to the preferred embodiment described above.
In the switching power supply unit and the electronic apparatus of preferred embodiments of the present invention, incorrect operation caused by electromagnetic waves emanating from cellular phones and other electronic apparatuses is prevented.
Other features, elements, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments thereof with reference to the attached drawings.